Behavioral, mental status and often fatal central nervous systems disease are commonly encountered in AIDS patients. These symptoms are often the result of a gradual or sometimes more acute encephalitic process. Diagnosis is problematic at best, brain biopsy being requires for definitive diagnosis and treatment. In AIDS patients toxoplasmosis encephalitis is due to the reactivation of latent (Chronic) disease involving the transition of the cyst (bradyzoites) to the active replicating form (tachyzoites). Nothing is known about stage specific antigens, how the transition form one to the other stage or what mediates this transition. The focus of this proposal is the isolation, identification and examination of stage specific gene products and the subsequent development of new diagnostic reagents based on these gene products, to allow the non-invasive diagnosis of encephalitis in AIDS patients. In addition, knowledge about stage specific genes in this parasite should prove useful for the development of vaccines and new therapeutic agents. To this end we have already generated a tachyzoite derived monoclonal library, cDNA in lambda gtll and genomic library and begun to employ these reagents in the diagnosis of experimental murine encephalitic disease utilizing DNA hybridization and antigen detection techniques. We plan during the course of this proposal to develop a bradyzoite derived monoclonal and cDNA library and to screen these together with our already developed tachyzoite reagents to identify stage specific gene products. Such stage specific reagents will be used in the development of diagnostic tests. In addition, we plan to increase the sensitivity of our diagnostic tests by utilizing the polymerase chain reaction for DNA probes and the avidin-biotin and /or NADP-alcohol dehydrogenase techniques for our antigen assays. We believe these studies will help to eliminate an important central nervous system complication of AIDS.